JP-H05-262118A, JP-H07-237419A, JP-H07-186660A and JP-H07-205629A disclose examples of a suspension controller configured to perform a preview control. In the suspension controller disclosed in JP-H05-262118A, a road surface sensor is provided on a front side of a front wheel of the vehicle, for thereby controlling, based on a value detected by the road surface sensor, damping characteristics of shock absorbers provided for the front and rear wheels of the vehicle. This suspension controller outputs a control command upon elapse of a delay time that is a length of time determined depending on (a) a distance between the road surface sensor and each wheel associated with the shock absorber that is to be controlled, (b) a running speed of the vehicle, and (c) a response delay time.
In the suspension controller disclosed in JP-H07-237419A, a front-wheel vertical-behavior sensor is provided to detect a vertical behavior of a front wheel, for controlling, based on a value detected by the front-wheel vertical-behavior sensor, damping characteristics of a shock absorber provided for a rear wheel of the vehicle. The damping characteristics of the shock absorber for the rear wheel is controlled through a control signal that is prepared based on a detected-value-basis control signal obtained on the basis of the detected value and also on a preview control signal obtained by delaying phase of the detected-value-basis control signal. A ratio of the preview control signal to the detected-value-basis control signal is made high when a running speed of the vehicle is lower than a predetermined value, and is made low when the running speed is higher than the predetermined value. When the running speed is higher than the predetermined value, the ratio is reduced with increase of the running speed. Consequently, the prepared control signal can be changed at the same phase as an actual vertical behavior of the rear wheel, thereby making it possible to satisfactorily suppress a vertical behavior of a rear-wheel side portion of the vehicle. It is known that the phase of the vertical behavior of the rear wheel is not delayed relative to the phase of the vertical behavior of the front wheel by a certain length of time (dependent on, for example, the vehicle running speed), but is somewhat advanced relative to a delayed phase that is delayed relative to the phase of the vertical behavior of the front wheel by the certain length of time. This is because the vertical behavior of the rear wheel is influenced by the vertical behavior of the front wheel due to rigidity of a body of the vehicle. It is further known that the phase of the vertical behavior of the rear wheel is advanced relative to the above-described delayed phase by an amount that is made smaller when the vehicle running speed is high than when the running speed is low. Consequently, the prepared control signal can be changed at a phase close to that of the actual vertical behavior of the rear wheel, by making the above-described ratio of the preview control signal to the detected-value-basis control signal lower when the vehicle running speed is high than when the running speed is low.
In the suspension controller disclosed in JP-H07-186660A, a sprung-portion acceleration sensor is provided in a front-wheel side portion of the vehicle, and a control signal for the rear wheel is prepared based on a value detected by the sprung-portion acceleration sensor. In this arrangement, the control signal is prepared by filtering the detected value by using a selected one of various filters that are different in phase characteristic. The one of the various filters is selected based on running speed of the vehicle. Consequently, no matter whether the vehicle running speed is high or low, the phase of the control signal can be made close to the phase of the actual vertical behavior of the rear wheel.
In the suspension controller disclosed in JP-H07-205629A, a preview total gain is determined based on longitudinal acceleration, lateral acceleration and running speed of the vehicle. In this suspension controller, the preview gain is changeable as needed, for example, when there is a delay in control response during running of the vehicle at a high speed or when there is a difference between path of the front wheel and path of the rear wheel upon cornering of the vehicle, such that the control output is reduced.